The present invention generally relates to implantable sensing devices, and more particularly to an implantable sensing device equipped with electrodes to provide a flow and/or chemical sensing capability.
Wireless devices such as pressure sensors have been implanted and used to monitor heart, brain, bladder and ocular function. With this technology, capacitive pressure sensors are often used, by which changes in pressure cause a corresponding change in the capacitance of an implanted capacitor (tuning capacitor). The change in capacitance can be sensed, for example, by sensing a change in the resonant frequency of a tank or other circuit coupled to the implanted capacitor. The circuit can be implanted with the capacitor in the patient, and equipped with an antenna, such as a fixed coil, that receives a radio frequency (RF) signal transmitted from outside the patient to power the circuit, and also transmits the resonant frequency as an output of the circuit that can be sensed by a reader outside the patient. This approach has been applied to monitoring joint pressure, orthopedic conditions, intracranial, and cardiovascular pressures. Capacitive sensors can also be coupled with resistive strain gauges, accelerometers and optical fibers to monitor bone integrity. The necessity for implantable sensors to operate at very low currents and power levels, as well as the desire to minimize the overall size of the implant, complicates the implementation of flow rate sensors and chemical sensors that rely on such conventional sensing devices as hot-wire anemometers, piezoresistive sensors, and other flow meters with moving parts.